This grant proposes to continue the study of human purine catabolism. The pathway catalyzes the final common degradation of nucleoside monophosphates to uric acid by a regulated reaction sequence. A rate-limiting step at the initial dephosphorylation step is proposed. Two major types of disorders are recognized in man. Blocks of purine nucleotide degradation lead to the accumulation of nucleosides and deoxynucleosides and result in immunodeficiency. Increased activity of purine nucleotide degradation may acutely result from tissue ischemia and a cascade of nucleotide breakdown to purine catabolic products. The latter may worsen the course of a disease process by making ATP generation and recovery less reversible. Using an integrated approach of basic biochemical techniques and clinical experiments we propose to study: (A) The regulation of purine nucleotide degradation by testing te hypothesis that 5'-nucleotidase activity is a rate-limiting reaction, further elucidating the control of this enzyme and evaluating the phosphorylation of adenosine and deosyadenosine; (B) The consequences of blocks of purine nucleotide degradation by characterizing the mechanism for inhibition of DNA synthesis by deoxyadenosine in cultured cells, directly testing the ribonucleotide reductase hypothesis in cells obtained from enzyme deficient patients, and determining the relevance of the S-adenosylhomocysteine accumulation hypothesis to cells of the immune system and to inhibition of DNA synthesis; (C) Increased activity of purine nucleotide degradation by testing the hypothesis that ATP depletion is responsible for hyperuricemia in glucose-6-phosphatase deficiency, measuring serum and urinary purines in cardiac surgery patients, and studying an hypoxia model in dogs; and (d) Treatment of purine nucleotide degradation by testing whether allopurinol blocks this pathway in man and cultured cells and continuing the observations that enzyme replacement therapy may reverse the block of this pathway in a patient with purine nucleoside phosphorylase deficiency. Elucidation of the biochemical regulation of purine nucleotide degradation and the molecular pathology underlying the disorders will lead to novel concepts of pathophysiology and to innovative therapeutic approaches.